1. Field of the Invention
This invention relates to power devices, modules and amplifiers. More particularly, it relates to biasing of a power transistor.
2. Background of Related Art
As is known, a very specific bias is required to ensure the linearity of Class-AB power amplifiers. However, the value of the required ‘optimum’ bias current varies somewhat with process and temperature.
To accommodate this variation, conventional devices provide a fixed bias (usually current) such that under nominal conditions, the best linearity is achieved. Ideally such conventional device would be maintained in an environmental condition to maintain it as close as possible to the nominal conditions (e.g., at a given temperature). However, as is appreciated by the present inventors, we do not live in an ideal world. As such, conventional devices nevertheless have some amount of degradation due to process and/or temperature variations.
Such fixed Class-AB bias typically results in sub-optimum linearity for any condition other than exactly the nominal condition. It is marketable and very desirable to meet or exceed certain given linearity specifications over a broad temperature range. Thus, any sensitivity of the linearity of a power device such as a power transistor to temperature directly translates into a potential reduction in the product yield.
There is a need for a power device having great linearity over a broad range of temperatures, particularly a Class AB type power device.